not popular but best:
Fallout, arcanum, KotOR, Vampire the masquerade, Fable, NWN...
if you dont like RPG;

if you like FPS:
Half life 1, all expansions, HL2, F.E.A.R...

if you like RTS:
Sudden strike, blitzkreig, Empire earth2, Starcraft

if you like Sports games:
Pro Evolution Soccer

if you like Simulation games:
Flashpoint...

that'll help ya:D

rip8fan1

Other popular games/series not mentioned:

The Sims Series
Myst Series
Warcraft/Starcraft Series
Ultima Series
Wizardry Series
Bards Tale Series
Roller Coaster Tycoon Series
Doom Series
Might And Magic Series (Including the Heroes of Might and Magic series)
Everquest (on-line)
Elder Scroll Series
Civilization Series
Age of Empires Series